Restoring a System 80 / Video Genie/ PMC-80 Part 4: Another cassette issue fixed.
This is part four of a restoration project which started with a non-working System 80. So far I had managed to fix a video issue, repair the cassette recorder (although not completely it seems..read on!), given the system a new case and added a switch toggle between the internal and external tape deck.
At the end of these fixes I thought I'd pretty well finished.
That is before I went to sell it!
I put the new slick-looking machine on our local auction site Trade Me advertising it as a fully-working unit. To my dismay halfway through the auction a previous problem came back to haunt me. Sometimes during PLAY there was simply no playback signal? I'd picked this up before but it seemed to have mysteriously disappeared. Now it was mysteriously back!
I couldn't pull the machine from the auction, so I simply made potential bidders aware of the issue. Much to my relief the machine didn't sell in the end. I wanted this machine to be up and running 100%.
I swapped the faulty deck with a known good one from another machine and then found everything was fine. This told me the problem lay with the cassette circuitry itself, rather than the digital one that fed into it.
Recording was OK in every instance; the problem only showed up during playback.
One of my fellow vintage computing enthusiasts, Andrew Joll, was knowledgeable regarding cassette recorders and I requested his help. After viewing the circuit diagram he felt it would be worth replacing a few of the capacitors which could directly affect playback (namely C5, C7 and C9) and maybe even the LM324 quad op-amp if that didn't work. One of the clues to capacitor issues was a signal burst (as seen on the meter) which occurred just as PLAY was initiated, even over a blank area of tape. Andrew called this a "bump".
The circuit board was easy to get to so after a quick trip to Jaycar to get the capacitors, I replaced the possible suspects (Fig 1).
Figure 1. The red arrows show the three capacitors replaced
I re-assembled and tested by loading in a tape 11 times in a row. Playback worked perfectly. Not only was there now a consistent signal, but the the meter was steadier than it had been before. Also, there was no sign of the "bump' on PLAY that had typified load sessions prior to the replacements.
Figure 2. Program loading now showing a strong, consistent signal
I think it's finally fixed.
In part two of this restoration blog series, I mentioned this problem. I thought then that perhaps it had fixed itself? This proved to be wishful thinking. I'm actually glad it did rear its ugly head again, as it gave me some impetus to get it fixed. Unlike before, I'm now confident this issue is sorted. Many thanks to Andrew Joll, for helping with this one.
Finished? Not quite. I decided I needed to do one final thing. The case had two holes for microswitches. I'd installed one to swap the cassette lines over. Why not fill the other with a switch which would allow full sound compatibility with TRS-80 software!
This is detailed in part 5...
27th July, 2010